The present invention relates to latent curing agents and accelerators for epoxy resins, especially one-component epoxy compositions. xe2x80x9cLatentxe2x80x9d curing agents are those curatives that in a formulated system remain inactive under normal ambient conditions but react readily with epoxy resin at elevated temperatures. xe2x80x9cAcceleratorsxe2x80x9d are those materials that accelerate the reaction between epoxy resin and a curing agent. xe2x80x9cOne componentxe2x80x9d epoxy compositions are typically a blend of epoxy resin, curing agent and accelerator as well as additives and fillers.
Current one-component epoxy compositions may contain an acid anhydride as a latent curing agent. These compositions offer excellent shelf life but require very high temperature to cure. Accelerators such as imidazoles can be used with acid anhydrides to increase reactivity, however such accelerators adversely affect shelf stability of compositions.
There is a need for a one-component 100% solids epoxy composition which is cured by acid anhydride and offers a good balance of low-temperature cure and shelf stability.
U.S. Pat. No. 3,329,652 discloses curing polyepoxides with acid anhydrides using imidazole salts as activators for the acid anhydride.
U.S. Pat. Nos. 3,356,645 and 3,418,333 disclose curing polyepoxides with imidazole salts.
U.S. Pat. No. 3,746,686 discloses curable epoxy resin compositions comprising a polyepoxide and a salt of a polycarboxylic acid or anhydride and an imidazole.
U.S. Pat. No. 3,755,253 discloses catalyzing the diaminodiphenylsulfone cure of polyepoxides with an imidazole salt.
T. Kamon, et al, xe2x80x9cCuring of Epoxy Resins. VI. Curing of Epoxy resins with Acid Salts of Imidazolesxe2x80x9d, Shikizai Kyokaishi (1977), 50 (1), pp 2-7 discloses the study of the curing of epoxy resins with alkyl carboxylic acid and phosphoric acid salts of some imidazoles.
JP 58083023 describes a latent epoxy hardener prepared by placing 2-hepta-decylimidazoline in water, adding orthophosphoric acid, mixing for 10 minutes, filtering and drying in vacuo. xe2x80x9cEpoxy Dispersion In Adhesive Applicationsxe2x80x9d, Adhesives Age, May 1995, pages 34-37, discloses the use of 2-methylimidazole and dicyandiamide in water-based epoxy compositions.
The present invention is directed to 2-phenylimidazole phosphate salt and its use as an accelerator for acid anhydride latent curing agents in one-component heat curable epoxy compositions. The salt is the reaction product of 2-phenylimidazole and phosphoric acid.
The reaction of 2-phenylimidazole with phosphoric acid, especially in equimolar amounts, affords the dihydrogen phosphate salt, or biphosphate salt, of the following structure A 
The invention provides:
an accelerator for acid anhydride latent curatives in heat cured epoxy compositions; and
one-component 100% solids epoxy compositions comprising 2-phenylimidazole phosphate salt, acid anhydride and an epoxy resin which offer a good balance of low-temperature cure and extended shelf stability.